1. Field of the Invention
The present invention relates to a bearing structure, in particular, a bearing structure for a camshaft in which a counterbore or a spot facing is formed to a part of a bearing surface of a lower journal.
2. Description of the Related Art
As shown in FIGS. 4 and 5, with regard to a camshaft 2 supported on a cylinder head 1 of the OHC engine, the camshaft 2 is clamped between a lower journal 3 provided in the cylinder head 1 and an upper journal (not shown) fixed thereon, and is rotatably supported by them. A plurality of lower journals 3 are aligned in an axis direction of the camshaft at certain intervals.
Each lower journal is supplied with oil from cylinder head side to lubricate interface between the lower journals and the camshaft. It is also known that only one lower journal 3 can be provided with oil and the oil is in turn provided to other lower journals via the camshaft 2. The cylinder head 1 is secured onto the cylinder body by head bolts (not shown) penetrating the cylinder head 1.
In recent years, it is required that an engine is smaller and lighter but more powerful. Therefore, bore pitch between cylinders becomes narrower, and the number of head bolts and valves becomes more. For this reason head bolts are close to the lower journals 3, and thus counterbores for head bolts interfere with bearing surfaces of the lower journals 3 in the cylinder head 1. This type of the cylinder head is known as described in paragraph 0006 and 0007, and FIG. 21 of Japanese Patent Application published by No. 8-218836, for example.
In this type, as shown in FIG. 6, while a counterbore face 4 for the head bolt is formed in the cylinder head 1, a cutting tool is moved down from above the cylinder head 1 to the level of the counterbore face 4 for the head bolt. In this process a part of the bearing surface 5 is cut off at where the route of the cutting tool overlaps with the bearing surface 5, thereby the counterbore 6 with a sharp edge is formed in the lower journal 3.
In FIG. 6, an upper end surface of the cylinder head is depicted by hatching. Numeral 4 shows the counterbore face for the head boils which is lower than the bearing surface 5. Numeral 7 shows a hole into which the head bolt is inserted. Numeral 8 shows a hole into which an injector is attached. Numeral 9 shows a female screw into which a bolt to secure the upper journal is engaged. X shows a hole for an inlet or exhaust valve stem.
Meanwhile, a connection part between the counterbore 6 and the bearing surface 5 is formed with a sharp, arc-of-circle like knife edge 10 as shown in FIGS. 7 to 9, because the cutting tool cut off a part of the bearing surface 5 when the counterbore face 4 is machined by the cutting tool in the cylinder head 1.
This knife edge 10 contacts with the surface of the camshaft 2 at an angle, then wipes off the oil from the surface of the camshaft 2 (knife edge function), because its relative contact point to the surface of the camshaft moves in turn in the width direction of the bearing surface 5 as the camshaft 2 rotates. This may cause lubrication failure against the camshaft 2.